Method for producing circuit board assemblies using surface mount components with finely spaced leads

ABSTRACT

Where an electrical connection is needed between an electric circuit on a substrate and a component with very finely spaced leads, pads are formed on the substrate at points where such connections to the circuit are to be made. A solder paste is deposited using a particular, described stencil having a thickness and apertures with specific tolerances. The component is positioned so that its leads to be attached are contiguous with corresponding pads, and the electrical connections are completed by reflowing the solder paste forming consistent and reliable electrical joints of solder alloy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention, generally, relates to a method for producingcircuit board assemblies with very fine pitch leads of surface mountcomponents connected to pads on the circuit board and, moreparticularly, to pad geometry, solder volumes used in such circuit boardassemblies, and stencil geometry used to supply solder paste.

2. Background

Recently in the manufacture of circuit boards and cards, the making ofelectrical connections between chip carriers and circuit boardsubstrates has encountered problems, and solutions have been evasive.

The increasingly fine pitch of component leads is causing acorresponding increase in the number of open circuit defects foundduring test of the circuit boards. Investigations indicate that thecause is from a failure of some leads to become connected, that is, someleads do not make contact with the solder on some of the pads.

The planarity of the attachment site and the relative planarity of theleads must be held to deviations less than the solder paste height orresulting solder metal height on the pads in order to reliably attachleaded surface mount components.

The clearest solution to this problem is to increase the volume ofsolder on the pads in order to increase the solder height during reflow,but when tried, such increase has produced other problems. It was foundthat the increase in the solder volume caused a type of defect called"solder bridge", because of the very close spacing between the pads.

On very elongated solder pads, too much solder causes solder bulgeswhich join together to form bridges between the pads. Thus, soldervolumes have been kept sufficiently low to prevent any such bulging.Also, the length-to-width ratio of solder pads has been kept low toavoid such bridging.

The use of solder pastes at all has been questioned in the surface mountassembly of fine pitch packages also because of increased demands onpost attachment rework requirements to remove solder bridges. However,an increase in the quantity of the solder paste is not the sole cause ofsolder bridge defects, but rather, the dimensions of the pads areinvolved also.

This problem does not exist only during the assembly process, butrather, it arises any time when connecting a fine pitch lead with a padon a circuit card, whether it is during the initial assembly or duringre-work.

3. Description of Background Art

U.S. Pat. No. 5,385,290 to Degani granted Jan. 31, 1995, describes arelationship between solder quantity and particle size and suggests animprovement is available through a use of a particular size of solderparticles.

U.S. Pat. No. 5,346,118 to Degani et al. granted Sep. 13, 1994,describes a method of forming solder bumps on metal pads using aspecific stencil configuration, and suggest that solder bumps thusformed will produce reliable solder joints.

U.S. Pat. No. 5,180,097 to Zenshi granted Jan. 19, 1993, describes amethod for mounting a component on a circuit board using strips ofsolder applied to a group of pads that are arranged to correspond withthe leads on a component.

An IBM Technical Disclosure Bulletin, Volume 37, No. 06A dated June,1994, describes a method of depositing solder paste on circuit cardsthrough the use of stencils with particularly defined apertures fordepositing smaller particle size solder pastes.

A Technical Paper, IPC-TP-901, by Morris, dated Sep. 5-7, 1990, entitled"Characterizing Solder-Pastes For The 1990s", describes results ofvarious tests of solder pastes and their relationships with stencils,suggesting a need for better control over the manufacturing process.

U.S. Pat. No. 4,998,342 to Bonnell et al. suggests a process for surfacemounting components on a circuit board.

Accordingly, the art identified above is hereby incorporated byreference.

The problem of developing open circuits during the connection ofcomponents to a circuit board or card appears to be of increased concerntoday as fine pitch components evolve into ultra fine pitch componentsand as the spacing between the pads becomes less and less.

Clearly, what is needed is a new and completely different approach todeveloping a solution. Such a solution is provided by the presentinvention, which is described in detail hereinafter.

OBJECTS AND SUMMARY OF THE INVENTION

An important object of the present invention is to provide a new andimproved method that provides optimum conditions during assembly of amulti-lead component on a circuit card so that solder paste will closegaps between contact pads on the card and the respective leads on thecomponent.

It is a principal object of the present invention to provide a processof attaching a component to a circuit board that minimizes the creationof an open circuit.

Another object of the present invention is to provide a method ofattaching a component to a circuit board that minimizes the developmentof short circuits.

Briefly, the method of the invention includes the steps of forming a padon a substrate at a point where an electrical connection is neededbetween an electric circuit of the card and a lead of an electroniccomponent or module. Then, a preselected solder paste is depositedthrough a stencil formed of a predetermined thickness with aperturesformed with specific, tight tolerances in order to achieve consistentand reliable results. Then the device that is to be connected ispositioned so that the lead that is to be attached is located contiguouswith the pad, and an electrical connection is completed by reflowing thesolder paste to form a joint of solder alloy.

Other features, objects and advantages of the present invention willbecome more readily apparent from the following detailed description ofthe presently preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, which includes FIG. 1a, FIG. 1b and FIG. 1c, illustrateselongated solder pads.

FIG. 2, which includes FIG. 2a, FIG. 2b and FIG. 2c, schematically showspads that are narrower than the pads that are illustrated in FIG. 1.

FIG. 3, including FIG. 3a, FIG. 3b, FIG. 3c and FIG. 3d, illustratessolder pads with different volumes of solder.

FIG. 4 shows a cross section of the solder pads of the invention withsolder bulges.

FIG. 5 schematically illustrates an embodiment of the circuit board ofthe invention.

FIG. 6 illustrates a partial cross section of FIG. 5 through a componentto be attached, such as a quad flat pack.

FIG. 7 discloses another partial cross section of FIG. 5 through anothercomponent, such as a direct chip attachment module.

FIG. 8 illustrates a gull-wing lead of a surface mount component towhich solder has been applied to form solder bulges.

DETAILED DESCRIPTION OF THE INVENTION

When solder paste on an elongated pad is reflowed in the absence of alead from a device or component and the volume of molten solder issufficient, the solder metal will form a visible bulge that is raisedabove the surrounding molten solder level by as much as 0.0025 inch(0.063 mm) on a contact pad.

It has been discovered that when certain, quite specific solder metalvolume requirements are met, not only will the formation of these solderbulges be ensured, but the plague of open solder joints will be reducedsubstantially and solder bridging between adjacent leads is avoided.

In accordance with the present invention, the solder volume is inaccordance with the following relationship: ##EQU1## where: VM=Volume incubic mm of solder metal alloy,

W=Width in mm of contact pad, and

L=Length in mm of contact pad.

Preferrably, the width (W) of contact pads should be smaller thanindustry standard guidelines to avoid bridging. As indicated by theabove relationship, the required solder volume increases inversely tothe width squared of the contact pad. A reduced contact pad width (W)causes the solder alloy to be confined within a narrower space, whichtends to cause a relatively higher solder bulge to form during reflow.

The above identified relationship was developed specifically for asolder of eutectic Pb/Sn. For other solder alloys, similar relations canbe developed following the principles of the present invention. Surfacemount attachment is accomplished normally by depositing solder paste onpads through a planar metal mask or stencil. The solder paste is amixture of metal particles, organic vehicles and flux. Solder pastesused for SMT usually contain about 50% metal by volume or 90% metal byweight, such as Kester R244.

The following is the relationship for the volume of solder paste for the50% metal-by-volume: ##EQU2## Where: VP=Volume in cubic mm of solderpaste.

W=Width in mm of contact pad, and

L=Length in mm of contact pad.

To further ensure that the above solder paste volume is obtained, astencil with specific dimensions must be used. It is important that thesolder paste that fills each opening of a stencil will result indeposition of a precise volume on each pad to form a bulge of moltensolder on the pad during reflow to contact the corresponding lead andnot be excessive to cause bridging short circuits with other pads.

The following table gives example information on dimensions inmillimeters (mm) to ensure correct solder paste volume to cause theformation of the solder bumps:

    ______________________________________                                                     STENCIL   APERTURE WIDTH                                         PITCH  PAD WIDTH   THICKNESS   TOP   BOTTOM                                   ______________________________________                                        0.3    0.114-0.127 0.102       0.102 0.127                                    0.4    0.165-0.190 0.127       0.190 0.216                                    0.5    0.229-0.254 0.152       0.241 0.267                                    ______________________________________                                    

Stencil thickness and aperture size should be maintained precisely asindicated above for all components being assembled with circuit cardsusing the method of the invention. Stencil thicknesses may generallyvary by about 0.025 mm and aperture widths by 0.025 mm as long as thefinal form is experimentally verified to produce bulges withoutbridging.

Moreover, the apertures should be smooth and tapered to provide a cleansolder paste release onto the contact pads of the circuit card tomaintain precise deposit volumes. With the small dimensions involvedhere, non-tapered apertures do not provide a high degree of reliabilityin solder paste release.

Also, the material of the stencil affects release of paste, andstainless steel is the preferred metal for the stencil surface. Thestencil apertures must be kept within tight tolerances and must besmooth and tapered. Therefore, a laser-cut stencil is preferred toachieve the reliability contemplated by the present invention.

While a plastic squeegee, such as one formed of a hard polymer, may workunder some circumstances, too frequently they fail to providesufficiently small volume tolerences because of the relative softness ofa non-metallic material in a situation involving the extremely smalldimension of stencil apertures. It has been discovered that a metalsqueegee for screening solder paste in such fine pitch componentassemblies will provide a high degree of reliability.

FIG. 1 shows molten eutectic Pb/Sn solder metal 100 on copper pads 102.The pads are sufficiently wide that they do not form bulges.

FIG. 2 also shows solder alloy 104 on copper pads 106. In this view, thepads are sufficiently narrow relative to the length of the pads andsufficient solder volume is deposited so that bulges form, but the widthis sufficiently high in relation to the pad-to-pad spacing that, as soonas the solder bulges on adjacent pads develops, bridging accurs.

FIG. 3 illustrates the solder configuration on the pads of the inventionas solder volume increases from FIG. 3a to FIG. 3d. FIG. 3a illustratesa volume of solder 150 that is not sufficient to form bulges, and inFIG. 3b, an embodiment of the invention is illustrated in which thevolume of solder 152 is sufficient to form bulges 154 on some of thepads.

The bulges tend to form somewhere in the middle third of the pad. Thepads are sufficiently narrow relative to the pad-to-pad pitch that thebulges which form on adjacent pads do not bridge. In FIG. 3b, the volumeis sufficient to significantly reduce the number of opens between thepads and the leads compared to the solder volume shown in FIG. 3a.

In the solder volume embodiment of the invention of FIG. 3c, the volumeof solder 156 is sufficient to form solder bulges 158 on all of thepads, thus minimizing the number of opens. The solder volume embodimentof FIG. 3c is the preferred embodiment.

Preferrably, the volume of solder in this embodiment is increased toabove that required to form bulges on all pads in order to increase theheight of the bulges, preferrably enough to produce average bulges ofaround 0.04 mm above the other solder on the pad.

In another embodiment of the invention, the volume of solder 160 in FIG.3d is higher than the volume in FIG. 3c, and some of the bulges formbridges 162 when there is no lead over the pads. If there are leads overthe pads, then molten solder tends to flow onto the leads, and even withthe volume of FIG. 3d, bridging will not occur between joints.

The volume of solder which would cause bridging between connectionjoints is not illustrated, but it would be significantly higher than thevolume shown in FIG. 3d.

FIG. 4 illustrates a cross section of the solder pads of FIG. 3c andshows an embodiment of the invention in which pads 170 are sufficientlynarrow relative to the pad pitch that solder may bulge without bridgingeven when there are no leads. Preferrably, solder mask 172 extends onthe circuit board surface around the rows but not between the individualpads in the rows.

FIG. 5 illustrates an embodiment of the invention in which a circuitboard 200 includes various surface mount components 202, 204, 206 and208 attached to a circuitized organic substrate 210. The substrate maybe a rigid planar substrate of polymer and reinforcing fibers or aflexible substrate of polyimide film and patterned metal film layers.The components are attached to the pads of the invention using thesolder volumes of the invention.

FIG. 6 shows attaching a component 202, such as a quad flat pack, to thecircuitized substrate 210 using pads 212 and 214 of the invention andsolder volumes 216 and 218 of the invention. The advantage of theinvention is illustrated by showing the lead portion 220 sufficientlyabove the level of the lowest leads connecting the component 202 thatthe solder would not have connected to portion 220 of the lead, if thesolder volume of the invention had not been used. Portion 220 of thelead is parallel to the pad and typically shorter (e.g., pad lengths60-80 mils and lead foot length 20-40 mils).

FIG. 7 illustrates coupling a component 204, such as a direct chipattachment module (DCAM) to the circuit board substrate 210 using pads232 and 234 of the invention and solder volumes 236 and 238 of theinvention. The leads 240 and 242 of the DCAM are pads on the bottom ofthe module that typically correspond to the configuration of the circuitboard pads for a quad flat pack.

Solder can be deposited on the DCAM pads by plating, contact with amolten solder ware, or by screening into a stencil in addition to or inplace of solder on the circuit board. The bumped solder pads of theinvention can be either on the DCAM pads or on the circuit board pads.

The advantage of the invention is illustrated by the lead 240 above thelevel of the lowest other leads connecting the component 204 so that thesolder does not connect between the pad 232 and the lead 240 when thesolder volume of the invention is not used. Preferrably, the lead 240 isa copper pad with the same dimensions as the pad 232.

Alternately for this type of component 204, the solder can be depositedon the leads of the component by screen printing solder paste asdescribed above or by otherwise depositing solder on the leads with thevolume of the invention to provide the solder to bulge from the leadand, thus, connect to copper pads 232 and 234 with respective leads 240and 242.

FIG. 8 shows a specific embodiment of the invention in which a gull winglead 250 of component 251 has a connection portion or foot 252 ontowhich solder 254 is deposited. Other portions of the lead can be coatedwith material 256, such as metal oxide or organic solder mask, which isless wetable by solder than the connection portion of the lead.

The solder can be deposited on the lead by screen printing paste or byplating or by dipping in liquid solder. If screen printed, the paste isreflowed before attachment in order to form a solder alloy.

Wiring layer 258 is deposited on dielectric surface 260 and includes arow of pads 262 and conductors 264 connected to the pads. The pads canbe coated with a thin layer of tin, solder or organic protective coating266.

The conductors and dielectric surface (except between the pads in therow) are coated with solder mask 268 which is highly non-wetting tosolder. The pads and/or the leads can be coated with flux 270 forimproved solder wetting between the pad and connection portion of thelead. Preferrably, the flux is sticky and is a no-clean flux.

When the component is reflowed before placement, the solder metal formsbulges 272. When placed on the pad 262 before reflow, the solder willbulge until it touches the pad and, then, flows between the pad 262 andthe lead 250 to form a solder joint leaving no visible bulge.

The invention has been described in substantial detail with reference tothe presently preferred embodiment. It will be understood by thoseskilled in this art that changes and modifications may be made, but allsuch changes and modifications are within the true spirit and scope ofthe invention which is defined by the appended claims.

What is claimed is:
 1. A circuit board, comprising:an interconnectstructure defining a plurality of component attachment sites,including:one or more attachment sites for surface mount components,each site including at least two parallel linear rows of metalinterconnection pads defining a rectangle, with the pads elongated in adirection lateral to the direction of the rows, and in which the ratioof width-to-length of the pads is low so that, when no component leadsis present during reflow and when solder volume is high, the solderforms bulges extending from surrounding solder; conductor wiresconnected to the pads and extending between the attachment sites; asurface mount component positioned at the surface mount attachment sitewith leads corresponding to the pads in the rows and positioned abovethe pads; and solder joints extending between pads and correspondingleads with a high solder volume so that, when no leads are presentduring reflow, bulges form extending from the surrounding solder bumpconsistently near the center of each of the pads, and sufficientlynarrow that the bulges form no solder bridges between joints.
 2. Thecircuit board of claim 1, in which:at least one of the surface mountsites includes two pairs of rows, the rows of each pair being parallel,each pair being perpendicular to the other pair, the rows of pads definea square; the center spacing of pads along the rows of the surface mountsite is 0.5 mm, or less; the product of the volume of the solder on eachpad divided by the length of the pads and divided by the width squaredof the pads is from 0.3 to 0.5; the solder volume is sufficient that,when no component leads were over the pads, the solder bulge is morethan 0.04 mm above the level of the solder over the remainder of thepad; the pads are at least twice as long as they are wide; the circuitboard includes exposed copper which is not coated with solder; and thewidth of the pads is less than half of the center-to-center spacing. 3.The circuit board of claim 1 in which the center-to-center spacing ofthe pads in the rows is approximately 0.4 mm or less.
 4. The circuitboard of claim 1 in which the center-to-center spacing of the pads alongthe rows is approximately 0.3 mm or less.
 5. The circuit board of claim1 in which the volume of the solder metal on each pad divided by thelength of the pads and divided by the width squared of the pads is from0.3 to 0.5.
 6. The circuit board of claim 1 in which the volume of thesolder metal on each pad divided by the length of the pads and dividedby the width squared of the pads is approximately 0.4.